1. Field of the Invention
The present invention relates to a failure diagnosis device and a failure diagnosis method for an inverter which are adapted to diagnose whether a failure has occurred in an inverter that drives a motor.
2. Description of Related Art
As a conventional failure diagnosis device and a conventional failure diagnosis method for an inverter, there has been known a technique whereby to diagnose a failure of an inverter on the basis of a resultant voltage obtained by combining the output voltage of each phase of the inverter, which is taken as the voltage of a virtual neutral point corresponding to the neutral point of a motor when switching elements of the inverter are PWM driven (e.g. refer to Japanese Patent Application Laid-Open No. 2006-81327). According to the technique, the output voltage of each phase of the inverter has a rectangular wave shape, so that a signal of the resultant voltage is passed through a low-pass filter, the cutoff frequency of which is lower than a PWM carrier frequency, thereby to extract a DC component from the signal of the resultant voltage, and the value of the DC component is used to diagnose whether a failure has occurred in the inverter.
Inconveniently, however, there is a time delay until the signal of the resultant voltage which has passed through the filter converges to a certain level. Hence, if an ON-fixed fault occurs in at least one of switching elements on an upper arm side and switching elements on a lower side, then an excessive short-circuit current may be generated between a power source and the ground through the inverter until it is at least diagnosed that a failure has occurred in the inverter. This may induce the occurrence of a secondary failure in the entire inverter.